Variable area fan nozzle with wall thickness distribution

ABSTRACT

A method of controlling flutter of a fan nozzle includes, for a given design of a fan nozzle, determining a vibration mode that causes a flutter characteristic of the fan nozzle and, in response to the determined vibration mode, establishing a wall thickness distribution of at least one wall of the fan nozzle to include local thick portions and local thin portions that alter the flutter characteristic.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure is a Division of U.S. patent application Ser. No.13/363,219, filed Jan. 31, 2012.

BACKGROUND

The present disclosure relates to gas turbine engines and, moreparticularly, to a variable area fan nozzle of a gas turbine engine.

A typical gas turbine engine includes a fan section that is driven by acore engine. The fan section drives air through an annular bypasspassage. The air is discharged through a fan nozzle. In some designs,the fan nozzle is moveable to selectively change a nozzle exit area ofthe fan nozzle and influence operation of the fan section, for example.

SUMMARY

A method of controlling flutter of a fan nozzle according to an aspectof the present disclosure includes, for a given design of a fan nozzle,determining a vibration mode that causes a flutter characteristic of thefan nozzle and, in response to the determined vibration mode,establishing a wall thickness distribution of at least one wall of thefan nozzle which includes local thick portions and local thin portionsthat alter the flutter characteristic.

In a further non-limiting embodiment of any of the foregoing examples,the vibration mode includes a vibration frequency and a strain mode thatis selected from a group consisting of bending strain and torsionstrain.

In a further non-limiting embodiment of any of the foregoing examples,the flutter characteristic includes at least one of an amount of flutterand a location of flutter.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the disclosed examples willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 schematically illustrates an example gas turbine engine.

FIG. 2 illustrates a perspective view of the gas turbine engine of FIG.1.

FIG. 3 illustrates a perspective, isolated view of a variable area fannozzle.

FIG. 4 illustrates a cross-section through a variable area fan nozzle.

FIG. 5 illustrates a representation of a wall thickness distributionthat includes local thick portions and local thin portions.

FIG. 6 illustrates an example fiber-reinforced polymer matrix compositematerial of a variable area fan nozzle.

FIGS. 7A and 7B illustrate finite element analysis of fan nozzles undera bending strain mode.

FIGS. 8A and 8B illustrate another finite element analysis of fannozzles under a torsion strain mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates an example gas turbine engine 20, andFIG. 2 illustrates a perspective view of the gas turbine engine 20. Thegas turbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22 and a core engine CE thatincludes a compressor section 24, a combustor section 26 and a turbinesection 28 generally disposed along an engine central longitudinal axisA. Although depicted as a turbofan gas turbine engine in the disclosednon-limiting embodiment, it should be understood that the conceptsdescribed herein are not limited to use with turbofans as the teachingsmay be applied to other types of turbine engines including three-spoolarchitectures.

The engine 20 includes a low pressure spool 30 and a high pressure spool32 mounted for rotation about the engine central longitudinal axis Arelative to an engine static structure 36 via several bearing systems38. It should be understood that various bearing systems 38 at variouslocations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 thattypically couples a fan 42, a low pressure compressor 44 and a lowpressure turbine 46. In the illustrated embodiment, the inner shaft 40is connected to the fan 42 through a geared architecture 48 to drive thefan 42 at a speed different than the low speed spool 30, in this caseslower than the spool 30. The high speed spool 32 includes an outershaft 50 that couples a high pressure compressor 52 and high pressureturbine 54. An annular combustor 56 is arranged between the highpressure compressor 52 and the high pressure turbine 54. The inner shaft40 and the outer shaft 50 are concentric and rotate via bearing systems38 about the engine central longitudinal axis A which is typicallycollinear with their longitudinal axes.

A fan nacelle 58 extends around the fan 42. A core nacelle 60 extendsaround the core engine CE. The fan nacelle 58 and the core nacelle 60define a bypass passage or duct B therebetween. A variable area fannozzle (VAFN) 62 extends at least partially around the centrallongitudinal axis A and defines an exit area 64 of the bypass passage B.The VAFN 62 is selectively movable in a known manner to vary the exitarea 64.

The compressor section 24 moves air along a core flowpath forcompression and presentation into the combustor section 26, thenexpansion through the turbine section 28. The core airflow is compressedby the low pressure compressor 44 and the high pressure compressor 52,mixed and burned with fuel in the combustor 56, then expanded over thehigh pressure turbine 54 and low pressure turbine 46. The turbines 46,54 rotationally drive the respective low pressure spool 30 and highpressure spool 32 in response to the expansion.

In a further example, the engine 20 is a high-bypass geared aircraftengine that has a bypass ratio that is greater than about six (6), withan example embodiment being greater than ten (10), the gear assembly 48is an epicyclic gear train, such as a planetary or star gear system orother gear system, with a gear reduction ratio of greater than about2.3:1 or greater than about 2.5:1 and the low pressure turbine 46 has apressure ratio that is greater than about 5. Low pressure turbine 46pressure ratio is pressure measured prior to inlet of low pressureturbine 46 as related to the pressure at the outlet of the low pressureturbine 46 prior to an exhaust nozzle. It should be understood, however,that the above parameters are only exemplary.

Most of the thrust is provided through the bypass passage B due to thehigh bypass ratio. The fan section 22 of the engine 20 is designed for aparticular flight condition—typically cruise at about 0.8 Mach and about35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with theengine at its best fuel consumption—also known as “bucket cruise ThrustSpecific Fuel Consumption (‘TSFC’)”—is the industry standard parameterof lb m of fuel being burned divided by lb f of thrust the engineproduces at that minimum point. “Low fan pressure ratio” is the pressureratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”)system. The low fan pressure ratio as disclosed herein according to onenon-limiting embodiment is less than about 1.45. “Low corrected fan tipspeed” is the actual fan tip speed in ft/sec divided by an industrystandard temperature correction of [(Tambient deg R)/518.7)̂0.5]. The“Low corrected fan tip speed” as disclosed herein according to onenon-limiting embodiment is less than about 1150 ft/second.

FIG. 3 illustrates a perspective, isolated view of selected portions ofthe VAFN 62. As shown, the VAFN 62 is a bifurcated design that includesa first VAFN section 62 a and a second VAFN section 62 b. In general,each of the VAFN sections 62 a and 62 b are semi-circular and extendaround the central longitudinal axis A of the engine 20.

FIG. 4 schematically illustrates a cross-section through the first VAFNsection 62 a. It is to be understood that the geometry of the first VAFNsection 62 a is exaggerated for the purpose of this description and isnot a limitation to the disclosed geometry. It is to be furtherunderstood that the second VAFN section 62 b is of similar constructionand geometry as the first VAFN section 62 a. In this example, the VAFNsection 62 a includes a body 70 that extends at least partially aroundthe central longitudinal axis A of the engine 20. The body 70 includes aradially outer wall 72 and a radially inner wall 74 that together formthe overall shape of the body 70 and thus the VAFN section 62 a. In thisexample, the body 70 generally has an airfoil cross-sectional shape.That is, the walls 72 and 74 of the body 70 form a wing-like shape toprovide a reaction force via Bernoulli's principle with regard to airflow over the walls 72 and 74.

In this example, the VAFN section 62 a is a hollow structure. Thus, theradially inner wall 74 is radially-inwardly spaced from the radiallyouter wall 72 such that there is an open space 76 between the walls 72and 74. Optionally, the VAFN section 62 a includes supports 78 extendingbetween walls 72 and 74 from wall 72 to wall 74 to stiffen andstrengthen the structure.

In operation, the first VAFN section 62 a and the second VAFN section 62b are selectively moveable to vary the exit area 64 of the engine 20.For example, the VAFN sections 62 a and 62 b are movable between atleast a stowed position and a deployed position such that in thedeployed position a greater exit area 64 is provided.

Airflow through the bypass passage B flows over the radially inner wall74 and, at least when the VAFN 62 is in the deployed position, also overthe radially outer wall 72. The airflow over the VAFN 62 causesvibrations in the VAFN sections 62 a and 62 b. Depending upon, forexample, the weight of the VAFN 62, certain vibration modes (i.e.,frequencies), can cause the VAFN sections 62 a and 62 b to flutter.Flutter is an aeroelastic event where the aerodynamic forces due tovibration, in combination with the natural mode of vibration, produce asignificant and periodic motion in the VAFN sections 62 a and 62 b. Theflutter can, in turn, elevate stresses at certain locations, cause theVAFN 62 to contact the fan nacelle 58 or damage the VAFN 62. As will bedescribed in more detail below, the disclosed VAFN 62 includes astrategic wall thickness distribution to reduce flutter and therebyenhance the durability of the VAFN 62 and engine 20.

FIG. 5 shows a representation of a wall thickness distribution 80 of theradially outer wall 72, the radially inner wall 74 or both of the firstVAFN section 62 a. That is, the walls 72 and 74 may have equivalent orsimilar wall thickness distribution 80 or, alternatively, havedissimilar wall thickness distributions 80. In that regard, in oneembodiment, the radially outer wall 72 has a first wall thicknessdistribution and the radially inner wall 74 has a second wall thicknessdistribution that is different than the first wall thicknessdistribution.

The wall thickness distribution 80 is represented by a plurality ofthickness zones 82. As an example, the walls 72 and 74 are made of afiber-reinforced polymer matrix material and the thickness zones 82represent one or more layers or plies in a multi-layered structure ofthe material. In that regard, each of the layers or plies thatrepresents the thickness zone 82 is selected to have predeterminedthickness such that when the layers of all of the thickness zones 82 arestacked and formed into the wall 72 or 74, the difference in theindividual thicknesses of the layers produce local thick portions 84/86,and local thin portions 88/90.

In this example, each of the thickness zones 82 is represented as apercent thickness, X %, Y % or Z %, of a preset maximum thickness of thethickness zones 82. As an example, X % <Y % <Z %. In a further example,X % is less than 40%, Y % is from 40-60% and Z % is greater than 60%. Ina further example, the present maximum thickness of one thickness zone82 is 0.5 inches (1.27 centimeters) or less. In embodiments, thethickness of a layer or ply, and thus the percent thickness, isestablished by changing the fiber density, fiber volume percent or areaweight of polymer of the layer or ply. Alternatively, each layer or plyis made up of sub-layers or sub-plies, and the number of sub-layers orsub-plies is changed to alter percent thickness.

For a given location or portion of the wall 72 or 74, the overallthickness, as represented in FIG. 5, is determined by the sum of thethicknesses of the thickness zones 82 in the particular location. Thus,the local thick portions 84/86 have thicknesses represented at 84 a/86a, and the local thin portions 88/90 have thicknesses represented at 88a/90 a. That is, the thickness 84 a is the sum of the thickness zones 82(in the vertical column) of X %, Y %, Z %, Y % and X %. Similarly, thethicknesses 86 a, 88 a and 90 a are determined by the sum of thethickness zones 82 in the respective vertical columns at thoselocations.

In a further example, the local thin portions 88/90 have a minimumthickness, thickness 88 a, and the local thick portions 84/86 have amaximum thickness, thickness 84 a. The minimum thickness 88 a is 90% orless of the maximum thickness 84 a. In a further example, the minimumthickness 88 a is 80% or less of the maximum thickness 84 a. In anotherembodiment, the minimum thickness 88 a is 70% or less of the maximumthickness 84 a, and in a further example the minimum thickness 88 a is60% or less of the maximum thickness 86 a. Additionally, in a furtherexample, the arrangement of the local thick portions 84/86 and the localthin portions 88/90 with respect to location from the leading end to thetrailing end of the VAFN section 62 a is a repeating pattern orsymmetric pattern.

The individual thicknesses of the zones 82, and thus the local thickportions 84/86 and local thin portions 88/90, are selected to control aflutter characteristic of the VAFN 62. In one embodiment, for a givendesign of a fan nozzle, which may be a fan nozzle or a variable area fannozzle, a vibration mode is determined that causes a fluttercharacteristic of the fan nozzle. As an example, the fluttercharacteristic includes an amount of flutter, location of flutter orboth. The vibration mode, as used herein, includes at least one of avibration frequency and a strain mode, such as bending strain or torsionstrain. Thus, for a given vibration frequency and a given strain mode,the given design of the fan nozzle can be analyzed, such as by usingfinite element analysis, to determine one or more fluttercharacteristics of the fan nozzle.

In response to the determined vibration mode, the wall thicknessdistribution 80 is established such that the radially outer wall 72, theradially inner wall 74 or both include local thick portions 84/86 andlocal thin portions 88/90 that alter the flutter characteristic. Withoutbeing bound to any particular theory, at a given location, the localthickness of the respective wall 72 and/or 74 influences the fluttercharacteristic at that location. In general, at each local location, thelocal wall thickness is reduced or minimized to alter the fluttercharacteristic and thus also reduce or minimize the overall weight.

In a further example, the fiber-reinforced polymer matrix material ofthe walls 72 and 74 of the VAFN 62 are made of a multi-layeredstructure, wherein each layer includes unidirectionally oriented fibers.In one example, the multi-layered structure includes 0°/90°cross-oriented layers and +/−45° cross-oriented layers. As shown in FIG.6, the radially outer wall 72 in a further example includes a region 92of 0°/90° cross-oriented layers and a region 94 of +/−45° cross-orientedlayers. It is to be understood that the disclosed example is alsorepresentative of the radially inner wall 74.

FIG. 7A illustrates an example finite element vibration mode analysis ofa given VAFN section (V) that does not include the above-described wallthickness distribution 80, represented as a two-dimensional projection.At a given vibration mode frequency, the contours 96 represent regionsof differing strain energy. In this example, the strain energy is abending strain. In general, there is a relatively high amount of bendingstrain at a leading edge LE of the VAFN section (V).

FIG. 7B illustrates the first VAFN section 62 a with the wall thicknessdistribution 80, represented as a two-dimensional projection. As shown,there is less bending strain energy at the leading edge LE and thus lessflutter than in the given design (V).

Similarly, FIGS. 8A and 8B show the given VAFN design (V) and the firstVAFN section 62 a at a given vibration mode frequency under torsionalstrain. In the given VAFN design (V) shown in FIG. 8A, there is asignificant gradient of torsional strain energy from the leading edge LEto the trailing edge TE. However, as shown in FIG. 8B, the first VAFNsection 62 a that has the wall thickness distribution 80 reduces thegradient from the leading edge to the trailing edge. Thus, in theexamples shown in FIG. 7A and 7B, the disclosed wall thicknessdistribution 80 alters the location of the flutter characteristic, andin the examples shown in FIGS. 8A and 8B, the disclosed wall thicknessdistribution 80 alters the amount and location of the fluttercharacteristic.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

What is claimed is:
 1. A method of controlling flutter of a fan nozzle,the method comprising: for a given design of a fan nozzle, determining avibration mode that causes a flutter characteristic of the fan nozzle;and in response to the determined vibration mode, establishing a wallthickness distribution of at least one wall of the fan nozzle to includelocal thick portions and local thin portions that alter the fluttercharacteristic.
 2. The method as recited in claim 1, wherein thevibration mode includes a vibration frequency and a strain mode that isselected from a group consisting of bending strain and torsion strain.3. The method as recited in claim 1, wherein the flutter characteristicincludes at least one of an amount of flutter and a location of flutter.